In 1999, patients in the USA received 85,788,400 composite restorations and the numbers continue to rise. Dental composite restorations represent the most prevalent exposure to biomaterials designed to restore form and function of replaced tissues. As such, further improvements in these widely used materials will have a considerable health care impact. While current materials are generally good, dental composites still suffer from deficiencies that limit their utility and performance. These problems include: relatively low conversion leading to leachable monomer, which along with water uptake, provide a decline in mechanical properties over time;lack of toughness;and most significantly, excessive polymerization shrinkage that causes stress. Shrinkage and stress associated with polymerization mean that intact restored margins can not be achieved reliably. All these problems complicate the placement of restorations and limit their service life. In this project, we address these issues through the inherent versatility of polymeric materials that allows controlled formation of heterogeneous structures. Polymerization-induced phase separation will be used to create nano- to micro-scale heterogeneous polymeric materials with high conversion, improved toughness and notably, with minimal stress development due to a late-stage shrinkage recovery unique to this approach. The research plan provides for development of (i) all-methacrylate resin compositions that form heterogeneous polymers, (ii) controlled formation of heterogeneous interpenetrating polymer networks obtained by simultaneous free radical and cationic photopolymerization processes applied to mixtures of monomers with different reactive functional groups and (iii) introduction of novel surface treatments for reinforcing fillers that provide a complementary additional tailored interface in composite materials based on the phase-separating resins. The proposed program will clearly delineate the effective control parameters that can be used to manipulate the polymerization-induced phase separation process to produce low shrinkage and low stress polymeric materials with improved strength, durability and toughness relative to conventional homogeneous materials. These resulting composite formulations will be suitable as next generation, improved dental restoratives. This research will advance fundamental understanding of complex polymeric systems while providing short- and medium-term clinical benefits with restorative materials.